Be careful with the terms you use.  A "full pension" is not defined anywhere in the appropriate documents.  Any pension that has not been reduced (e.g. due to breaking terms of service, marital breakup) could be described as a "full pension" as you would get everything you are entitled to based on the formula:  2% X years of service X average of best five years" salary.  What I suspect you really meant was that should be entitled to an "immediate unreduced annuity" after 25 years, meaning you could start collecting a pension at that point (prior to that, you would have to take a deferred annuity or a return of contributions).  To maximise your pension, you need to serve at least 35 years.